The liberals are completely clueless on this issue. I know a woman who taught in an inner city school for a number of years. She told me that for the most part, the children of single moms in these schools cannot be taught much no matter how good the teacher. They simply do not want to learn. And she felt that most of the teachers in her school, at least, were reasonably competent. Until someone figures out how to change the culture in the minority community, nothing will ever change. When prominent blacks like Cosby try to point this out, they are shouted down.
And when conservatives point this out, they are criticized for actually arguing that kids CAN learn in public schools (if their home life encourages learning), or that public school teachers actually are mostly competent.